It was recalled that dynamic disorder in the high-temperature solid phases of these Zintl compounds was characterized by fast orientational motion of the polyanions, and coupled motions of the cations. In CsPb, these appeared to occur on a similar time-scale whereas, in NaSn, the orientational motions were more rapid. The cation motion in NaSn appeared to be of translational type, in which the Na+ ions migrated through the crystal along pathways that were parallel to the c-axis. In both cases, melting was characterized by the appearance of slow translational motions of the center of mass of the polyanions. The dynamic behavior of the ions was associated with marked increases in the electrical conductivity. Here, CsPb exhibited the behavior which was expected of a narrow-gap mixed conductor, while NaSn behaved like a wide-gap mixed conductor.

D.L.Price, M.L.Saboungi: Journal of Physics - Condensed Matter, 1997, 9[13], 2707-15